Coinwatcher, it is one thing to look for varieties. Many here have encouraged your searching out varieties.
It is quite another to list them for sale when you have no real idea of what the item is worth.
My advice above is to set the starting price low, describe realistically and see what happens. You are not even sure if the stamp is what it is. You are basing it on Ralph's comments and some un-named professional (can you tell us who this was?).
I totally disagree with your value. The stamp is not well-centered and has a dot the MAY indicate its plate position.
Your listing bothers me in many ways, and should really be removed so some unsuspecting buyer does not get taken.
First, your price is not realistic. There are other positions that are recognized based on a plate varieties/marks. This does not make them suddenly worth 100's of dollars.
Second, you quote Ralph Trimble's reply without any acknowledgement. I trust you got his permission to do so.
He also did not say it WAS that stamp, simply that it sounded similar based on an earlier description. Have you ever read Duckworth's book? There is a new one out now, totally revised (see VGG website).
Third, there are many gullible buyers on
ebay that often buy items for far more than they are worth (not that I think they will buy your stamp - hopefully not at that price).
How would you feel if some buyer did just that, then found out when it was time to sell that the stamp was worth a fraction of what they paid?
Do some of your own research, then you can price the item realistically.